1. Technical Field
The present invention relates to a light-emitting element, a light-emitting device, a display device and an electronic apparatus.
2. Related Art
An organic electroluminescent element (organic EL element) is a light-emitting element having a structure in which a luminescent layer including at least one organic layer that can emit light is disposed between an anode and a cathode. In this type of light-emitting element, electrons and holes are injected into the luminescent layer respectively from the cathode and the anode by applying an electric field between the cathode and the anode, and the electrons and the holes recombine with each other to form excitons in the luminescent layer. When the excitons return to the ground state, the energy is emitted as light.
The luminescent layer of a light-emitting element of this type may be formed by an ink jet method or any other liquid application (liquid phase process), as disclosed in JP-A-2009-280576. Since liquid application allows a pattern to be formed without using photolithography, the light-emitting element can be manufactured by a simple process, and the amount of material to be used can be reduced. These advantages of liquid application are markedly shown particularly in the process of forming a large number of light-emitting elements in a large panel.
In general, light-emitting elements formed through such a liquid phase process tend to exhibit a more significant rapid decrease (initial drop) in luminance with time in an early driving period than light-emitting elements formed through gas phase processes. In addition, the rapid decrease in luminance in a light-emitting element formed through a known liquid phase process may cause the emission color to change, and it is difficult to increase the life of the light-emitting element.
Probably, the cause of the luminescent color change is that, when the thickness of the luminescent layer is increased, the position and width of the luminescent region of the luminescent layer are changed, and that the optical path length in the luminescent layer is varied accordingly.
The change in the position or width of the luminescent region may be suppressed by reducing the thickness of the luminescent layer. However, if the thickness of the luminescent layer is reduced, excitons defining the luminescent region reach the layer adjacent to the luminescent layer (such as a hole transport layer or an electron transport layer). Consequently, the excitons are deactivated and the luminous efficiency is undesirably reduced. The luminance of the light-emitting element is thus reduced.
In addition, if the thickness of the luminescent layer is merely reduced, it is difficult to ensure an optical path length sufficient to produce efficient luminescence in the luminescent layer. This is also a cause of the reduction in the luminance of the light-emitting element.